Malignant transformation of cells by viruses often is induced by a virus-coded protein which plays no role in virus reproduction. Cells infected with the avian MC29 virus produce a protein, p110 gag-myc, containing a viral structural domain (gag) fused to a cellular domain (myc). We have found that the protein migrates to the nucleus soon after synthesis, and has a brief intracellular half-life. The p110 gag-myc exists in transformed cells in both monomeric and dimeric forms, similar to several other eucaryotic and procaryotic regulators of transcription or DNA synthesis. No cellular proteins were found consistently to associate with p110 gag-myc. The cellular homologue p55 myc also was found to exist in dimeric form, as was the avian myeloblastosis virus protein, p50 myb. Possible biochemical activities associated with dimeric forms are under investigation. In another series of experiments, morphological variants of cells infected with an MC29-related virus, MH2, were isolated. Whereas p55 myc was unable to be found in cells transformed with MC 29 or MH2 viruses, the p55 protein reappeared in the cellular variants, suggesting the possibility that in MH2-infected cells the viral myc proteins control the synthesis or modification of cellular p55 myc.